Resistance thermometer



Juanma@9 1948. mamma-a RES I S TANCE THERMOMETER Filed Jan. l1, 1944 AGENT Patented June 29, 1948 UNITED STATES PATENTHOFFICE RESISTANCE THERMOMETER George Kelnath, Larehmont, N. Y.

Application January ll, 1944, Serial No. 517,795

l Myjinvention relates to electric resistance thermometers and is related to the subject matter of l. my copendins application, Serial No. 428,028, tiled January 24, 1942, now Patent 2,365,706 granted December 26, 1944, of which this is a continuation-in-part.

-Electric resistance thermometers, comprising a thermoresponsive resistor as the temperature sensitive measuring organ, are usually connected with .an electric measuring circuit of the balanceable type. as represented by a Wheatstone bridge for instance. A variation f the thermoresponsive resistance disturbs the balance of the circuit, and the adjustment necessary for compensating the unbalance serves as a measure of the tempera- Yture to be determined.

. To realize a rugged construction of a resist- .ance thermometer which is capable or being read- .,ily-manufactured and of having a high degree of accuracy, the resistance thermometer should be designed on a low resistance basis. It is however undesirable to design the bridge circuit on a low impedance basis, such as would be required were a temperature'responsive element of low resistance placed directly in the bridge circuit, for then the resistance of lead connections in the bridge circuitwould be of a suillciently high relative order tobecome a source of error. Referring to these conditions, it is an object of my invention to atl'ord using a low-resistance temperature responsive element and yet also a measuring circuit having circuit elements of relatively high impedance in order to combine the desired advantages of a low resistance thermometer while avoiding the disadvantages of low impedance measuring circuits.

Another object of the invention is to provide r ,a resistance thermometer of improved mechanical and electrical construction and properties for operation with alternating current.

A further object of my invention is to devise a resistance thermometer which is compensated for ambient temperature variations so as to ailord an' improved degree of accuracy.

It is also an object of my invention to provide a resistance thermometer which is capable oi' .measuring higher temperatures than those for which the known thermometers of this type `were A normally applicable, or which has a considerably 2 Claim. (Cl. 'I3-382) ture heretofore measured by pyrometric thermocouple devices but affords higher measuring currents and voltages so as to produce for instance,

a greater torque in the indicating or recording instrument than that directly obtainable with thermocouples. In comparison with thermocouple instruments, my invention aims also at aiording an improved reading accuracy and the possibility of using a suppressed scale.

In order to achieve these and other objects, which will become apparent from the following description, I provide according to this invention a temperature responsive resistance element of relatively low resistance value with a step-up impedance transformer that has a low impedance winding connected with the resistance element and a high impedance winding to be connected with a measuring circuit. This resistance element and transformer are structurally combined to a single thermometer unit so that variations in resistance o1' the connecting conductors leading from this unit to the measuring circuit proper are largely prevented from aifecting the measuring result.

In conjunction with the latter aspect of my invention and in accordance with another one ,of its essential features, I provide a temperature responsive compensating resistor in proximity to .the transformer for reducing or eliminating the effect on the measuring result of any temperature dependent variation of the transformer, the compensating resistor being arranged so that it is subjected to the same temperature as the transformer.

The foregoing objects and features as well as other characteristics of my invention will be understood from the following description of the embodiments shown in the drawing in which Figure 1 is a partly structural and partly schematic illustration of a resistance thermometer according to the invention in conjunction with a measuring circuit suitable for its operation;

Fig. 2 shows a modied form of the thermometer alone; while Fig. 3 is a diagrammatic showing of a third embodiment in conjunction with a modied measuring circuit.

Referring to Fig. 1, the measuring system shown therein comprises a thermometer unit which, as a whole, is denoted by the` letter This unit contains a temperature responsive resistor r1 whose terminals are connected to the secondary l of a transformer Il having a primary 2 for connection with the measuring circuit the transformer primary 2 in series connection with a calibrating resistor R1. Another calibrating resistor Rz in series-connection with the above-mentioned compensating resistor r2 lies in a second arm of the network.. A third arm contains an inductance element L4 and the fourth arm of the network inclu-des, in series, an .inductance element La and a variometer Z3 c0mprising relatively angularly adjustable coils C and C. Theresistor R2 and inductance element L4 are standard parts but each -is preferably adjustable as is shown. The junction points 5 and 5' are interconnected by a zero branch which contains a reversible control means formed by a motor 6. As shown, an amplifier i3 may be interposed between the motor 6 and the zero branch proper. The motor 6 has a mechanical drive connection 'l (diagrammatically shown) to the variorneter Z3 and to an `indicating means Aherein simply shown as a pointer 8 and scale d', it being understood that the pointer is secured -directly to the movable coil of the variometer to move in unison therewith. Current is supplied to .the bridge circuit from an A.C. source I2 through leads 9 which are connected to the junctions i@ and I of the bridge circuit.

It will .be understood that when the bridge is balanced, junction points and 5' have the same potential and no current flows through the zero ,l

`branch so that the motor 6 remains at rest. However, when the bridge is unbalanced by a variation of the resistance r1 of the thermometer T, current will flow through the motor E andcause the motor to turn the movable coil of the variometer in such a direction as to rebalance the bridge circuit thereby restoring the Cur- ,rent through the motor to a substantially. null value. For a vprescribed-'characteristic of resistance r1 versus temperature and for alike characteristic of the inductance Z3 versus angular deflection of its movable coil, lthe angular deflections of the movable coil of the variometer to maintain the bridge balanced are directly proportional tothe temperature variations of the resistance r1 and will be indicated by the angular deflections of the pointer 8 relative to the scale 8. Hence, the adjustment of the indicating device 8, 8', automatically effected by the just-described circuit means, is indicative of the tem.-

perature to be measured.

' For best sensitivity, the arms of the bridge circuit should be substantially equal in impedance; while, to rule out the resistance of lead connections in the bridge circuit as a source of error,

the elements directly connected in the bridge circuit should have a. substantial value of impedance as of the order of 100 ohms or more. A re- Is istance thermometer having a resistance of 100 ohms at zero degrees centigrade cannot be readily made rugged as the wire constituting the resistance r1 must be very line. These difficulties are however largely overcome when the thermometer is made to have a low value of resistance, say of the order of only one ohm at zero 4 degrees C.; such a thermometer will withstand much higher temperatures, and the insulation requirements for the wire are then greatly relaxed. The resistance of the thermometer must however be stepped up to present the desired 100 ohms to the bridge circuit. This is done by means of the resistance matching transformer Il having an impedance ratio o! l to 100, or in other words a turns ratio of 1 to 10. The use of the resistance matching transformer ll limits the bridge to A.C. operation-that is, to being supplied with alternating current from an A.C. potential source, which may be an independent -source l2 as; aforementioned, or from a regular S0-cycle lighting circuit. A.C. operation has the advantage -of permitting a greater latitude of design of the 'bridge circuit in enabling the use "of either resistance, inductance or capacitance elements, and of permitting the use of an A.C. amplifier |13 (-diagrammatically shown) between the Junctions 5 and 5 and the motor 8, for .increasing the sensitivity of the recordenwhich amplifier is preferable to a D.C. amplifier be- .cause of its greater stability, Furthermore, A.C. operation has the highly important advantage. .of ruling out the need for any sliding contacts in the bridge circuit, as hasbeen explained, and of permitting the use of an A.C. motor as the null detector, which also maybe devoid of sliding contacts.

The transformer H is to have a relatively low order cf dissipationi. e., of copper (or resist,- ance) and iron losses, -For instance, the efi'ective losses in the transformer expressed in ohms should not be. greater Athan about live per cent oi' the active resistance of the thermometer. The losses in the transformer should lalso .not vary appreciably with temperature since the ,transformer and resistance r1 are placed adjacent to one another in one housingto obtain a low resistance connection between the resistance r1 and the primary of the transformer. In this case the transformer will be subjectedin substantial degree to temperature variations caused by `those to be measured, and must havefa high heat resistant insulation,I as of asbestos. to'withlstand high temperatures. I n` measuring the temperature within a furnace, for example, the temperature of the transformer may be as high as 150 C., for the head 'l5 will in'this case be mounted directly on the wall of the furnace.

My invention contemplates compensating for Avariations in the losses within thetransformei by the use of a compensating means located in juxtaposition to the transformer within the head 'I5. Unless an especially high degree o f accuracy in measurement is required, no compensation is required for the iron losses (from hysteresis and eddy currents) as these losses canbe reduced to a very low level by the use of low-loss nickel-iron alloys in the magnetic core of the transformer. .The copper losses need however to be compensated for. This purpose is taken care of by the above-mentioned resistor r2 in the head I5 of the `thermometer unit T. The resistor rz consists of copper and is in thermal contact with the trans'- former il. As this resistor r2 is series-'connected with vthe standardl resistance Re. between the junctions 5 and I0 and in a bridge arm adjacent to the arm of the transformer Il, it will serve to annui the effects in the bridge circuit of variations in the copper losses of the transformer Il, the value of the resistance n being appropriately selected to give the desired compensation. The resistance n of the thermometer fr :2| that extends through the end of not vary appreciably prises a coil of wire composed preferably oi platinum and made relatively heavy, say as large as .3 mm. in diameter. The use of such heavy wire provides a high degree of accuracy, for the accuracy increases with the diameter of the wire. In particular. when the wire has a large crosssectional area, it has less area of exposed surface per unit volume and is subject to less percentage contamination-that is, for a given volume of wire it will not alloy with other metals, will not adsorb and dissolve gases and will not acquire deposits of vaporized metals to the extent that it would were it made with a smaller area of cross-section.

Although the wire element constituting the resistance n is made heavy, to have it protected in every` practically possible way. vTo this end, it is encased in the tube il and mounted on a quartz tube I8 as is shown in Figure l. This quartz tube is telescoped over the end of a rod I'I which is held concentrically within the tube Il by a pair of spaced porcelain washers il an il' that embrace the outer lend Portion ofthe rod and nt the interior of the tube Il, the inner washer I8 being, for example, abutted against a shoulder I4' on the tube Il and the space between the washers being suitably filled as with a quartz sand Il. An auxillary support for the quartz tube I is formed by a cap member which nts over the end of the tube i0 and which has a central projecting post the tube Il and is welded thereto.

suitably secured to the open end portion of the tube is the head I0 aforementioned. This head has a reduced diameter portion Il' embracing the tube il and -threaded at 22 to facilitate connecting the thermometer to -a support. 'I'he part of the head i6 extending beyond the tube I4 is enlarged in diameter to receive the transformer and the compensating resistance rz. Closing the end of the head I0 is a Bakelite disk 23 which tightly fits the interior of the head.

To provide connections, between the resistance r1 and the primary of the transformer Il, having a magnitude of resist-ance negligibly small in relation to that of the resistance r1. one of these connections is made through the rod I1, the rod accordingly, being provided with a relatively large diameter as of the order of 4 mm. and the other lead connection is made through the cap 20 and outer tube Il. In order that what resistance these lead connections may have will with temperature, the rod H' and tube I4 are made of a. metal having a low temperature coemcient of resistance, preferably of the nickel-chromium alloy known under the name of Nichrome which has also the advantage of being a. poor heat conductor and of being thus unable to conduct the heat effectively to the head I5 and the transformer 'I'he lead connections of the secondary winding of the transformer to the bridge circuit are made through terminals 24 mounted on the Bakelite disk 20. Also, one lead connection of the compensating resistance r2 to the bridge circuit is made through a terminal 25 on the disk 23, the other lead connection of this compensating resistance being however made within the head |5 to one side of the secondary of the transformer il as is shown in Figure 1.

Certain advantages and preferable electrical shown in Fig. 1, are explained in my above-mentioned copending application, Serial No. 428,028.

there is yet need r now Patent No. 2,365,706, to which reference may be had, if desired. These other details of the exemplified circuit are not relevant to the present invention proper, and it will be understood that measuring circuits other than the one illustrated may be used in connection with thermometer units according to the present invention.

According to the embodiment shown in Fig. 2, a temperature responsive 5 resistor |30 (corresponding to resistor r1 of Fig. l). is mounted on e ceramic or quartz tube il 6 which is placed over one end of a central rod ill. One end of the resistor is attached to a metal cap |20 welded at tilt to the closed end of an elongated tubular enclosure ill. The other end of resistor |30 is in conductive connection with the rod Il. The tube ||4 contains at least one ceramic washer or spacer H8 and is filled with quartz sand III or the like finely divided filler of insulating material so as to reduce the gas spaces within the enclosure and improving the heat conductivity v between the enclosure ||4 and the resistor |00.

The enclosure I4 surrounds the resistor |30 as .closely as feasible in order to keep the heat capacity of the measuring end of the thermometer as low as possible without foregoing a good and permanent electric insulation of the resistor |30 or causing undue difliculties of manufacture. A spacing of about 1 millimeter between the enclosure ||4 and the resistor |00 is. as a rule, sui'licient and convenient.

The resistor |30 consists, for instance, of platinum. Its resistance, when cold, may be in the order of 1 ohm. In order to reduce its' inductance, and to eliminate the effects of magnetic stray ilelds, the resistor is designed as a bilar type winding. That is, one portion of it is wound in a direction opposite to that of the other portion.

If desired, the resistance winding |30 can be attached to the quartz tube, by providing the winding with a coating of quartz or other vitreous material and fusing this coating to the tube, although a procedure of this type is not necessarily required. The tube Hl and th'e rod ||1 consist preferably of metal Vof a lowv temperature coeiiicient of electric resistance, such as the above-mentioned nickel-chromium alloy Nichrome." The rod has one or several bends or turns |31 for compensating its thermal expansion. A heat resistant insulating seal is provided at |80. It consists of' vitreous or ceramic material and may form a gas-tight closure at this end of the tube III. The interior of the tube is evacuated or provided with an inert gas at low pressure to prevent corrosion of the resistor Ill at high temperatures.

When manufacturing the above-described high temperature portion oi the thermometer, the following procedure may be followed.

After mounting the quartz tube on the rod and attaching the resistor winding i30-to the rod and to the metal cap |20, the assembly thus formed is inserted into the metal tube ||4 so that the pin 52| of cap |20 passes through the correspending opening of tube H4. The pin |2| is then welded to the tube to obtain a gas-tight seal. Then the tube M4 is filled with quartz sand and provided with the washers ||8 and |30, th'e latter being ready for thc sealing operation and to this end provided, for instance, with a solderable metal glaze unless some other sealing technique is to be employed. The assembly is then placed into a furnace and heated at about 7 500' centigrade. lThe soldering or other sealing operation is performed at this temperature. Then, th sealed assembly is permitted to cool slowly, preferably while remaining in the furnace;

Thus, the air and oxygen contents within th'e sealed tube is rariiled since, after cooling to normal temperature (20 centigrade), the pressure within the tube is reduced to almost one-third of normal atmospheric pressure. Consequently, the remaining gas in the tube is relatively inert so that excessive corrosion of resistor |30 is avoided. At the same time, the reduced pressure at room temperature has the effect that only moderate interior pressures are obtained when th'e thermometer' is subsequently exposed to high operating temperatures. If desired, the pressure during the sealing operation may be reduced to a further extent by connecting the furnace 'with a vacuum pump; or an inert gas, such as nitrogen, may be substituted for the residual air in tube ||4 by means of the corresponding methods known in the manufacture of incandescent lamps or electronic tubes.

V"Referring now to further details of Fig.'2, the sealed end 'of tube Ill is attached to the neck 4|22 of a head portion H5 which forms a housing for the transformer and its appertaining terminals. The transformer has a three-legged laminated magnet core |33 seated on the bottom of the housing H5. The center leg of the core is provided with a low impedance secondary I consisting of a relatively heavy copper wire of relatively few' turns. A high impedance primary ISI of many turns of relatively fine copper wire is also placed on the center leg of the core. The secondary Ill'has its terminals connected with the rod ||1 and the tube ill respectively, while the primary |35 is connected to terminals |24 and |25 mounted on an insulating plate |23. A compensating resistor |38 (corresponding to resistor r2 in Fig. l) is also placed on the center leg of core |33 and consists of a bifllar winding which at one end is connected to the primary terminal |25 while the other end is attached to a separate terminal |28. The terminals are protected bya removable cover |21. The neck portion |22 of the thermometer head engages a bore |40 of th'e furnace wall Ill. As in the first described embodiment, the ratio of secondary to primary turns is, for instance, 1 to and-the resistance ratio, for instance, 1 to 100. The preferable rating of the compensating resistor |36 depends on the iron and copper losses of the transformer. Using a transformer of low iron losses, one may assume that the copper losses `and their temperature dependent changes occur virtually only in the 100 ohms primary circuit and amount to at most about 5 ohms. Hence, the resistor 5 ohms when cold, the actual value being, of course, subject to changes in accordance with the temperature responsive resistance changes in the transformer copper.

. .With respect to both above-described embodiments, the inductivity of the impedance matching transformer may usually be neglected as regards its effect on the balance of the measuring circuit connected to the transformer primary. That is, as long as the transformer inductance remains below 5% of the resistance-and this is normally the4 case--the error possibly introduced into the measuring result due to the transformer inductance remains below .1%. If lthe inductance lis higher, it is advisable to insertan auxil-4 |36 may be rated for lary oounter-inductanceof about the same uns# nitude in the adjacent arm of the measuring circuit (the armv of Rn in Fig. 1). This can be done by providing the compensating resistor n or Il. with a corresponding inductance value, i. e'. by using a wholly or partly uniiilar resistor; or by adding a separateinductance. Suchl a separate inductance may be placed into the measuring cir' cuit remote from the thermometer because this inductance may remain cold during the thermometer operation. An additional inductance of this type is shown in Fig. 3 and denoted by La, the circuit being otherwise similar to that of Fig. 1 as will be apparent from the corresponding reference characters.

Fig. 3 illustrates also some other modifications. For instance, the adjustable inductance Z: is dei signed for manual operation, a zero indicator I being arranged in the zero 'diagonal of the bridge circuit. The resistance thermometer T according to Fig. 3 is providedl with `an insulating protective tube lli consisting of a ceramic material', as used for pyrometer tubes. This requires an insulating seal |42- traversed by two conductors for connecting the resistor n with the 'transformer Il.

Resistance thermometers according to the invention are applicable for higher temperatures than those heretofore normally applied to'th'ermometers of this type. For instance, tempera.;l

- tures of about 500 C. up to 800 C. andmore can be measured in normal and continuous operation. In this respect, these thermometers are competitive to thermocouples. In favorable contrast to the latter, however, my resistance thermometers as disclosed afford a much higher torque in the indicator due to the fact that the available energy fis taken from an outside power source which may consist of the customary alternating current line and hence requires no batteries. As a matter of fact, a torque of, say, 10l times that afforded by a thermocouple is Areadily available. A 'thermometer according to my invention avoids also the disturbances or errrs encountered with thermocouples due to "cold ends" and permits' also the applicationI of any sup#i pressed scale', thereby indicating smaller changes of temperature and increasing the reading accura/cy.

'It will be obvious to those skiilled in the art; and is also apparent from the different modifie' cations exemplified in the foregoing, that-the' design of the thermometers may be varied with! out departing from the gi'st and essential char; acteristics of my invention. I, therefore, wish this specification t'o be understood as illustrative rather than in a limiting sense.

I claim as my invention:

1. An electric resistance thermometer for measuring high temperatures, comprisingA an' enclosure having anelongated and gas-tight tubular portion of refractory metal closed at one' 'of its ends, a head portion disposed at theothe'r end of said tubular portion, and hermetic fsealing means separating the interior lofsaid tubular portion from said head portion; a metalrod'extending coaxially within said tubularI portion over substantially the entire lengthof said tubular portion and through said sealing means into said head portion; ai' quartzlike body of insulat-l ing and refractory qualities carriedlby said'rod and enclosing an end portion thereof adjacent to the closed end of said tubular portion; a temperature-responsive resistance winding mounted' on said' body 'and electrically connected between' said tubular portion and said rod respectively; and electric circuit means'disposed in said head portion and electrically connected with said rod and said tubular portion for connecting said winding to a measuring circuit.

2. An electric resistance thermometer for measuring high temperatures, comprising an enclosure having an elongated metal tube closed at one of its ends and a head portion adjacent to the other end of said tube, a rod extending lengthwise within said tube; an insulating refractory body carried by said rod at an end portion thereof adjacent to the closed end of said tube; a temperature-responsive resistance winding of metal mounted on said body and electrically connected to said rod and said tube respectively so that said rod and said tube form connecting leads for said winding; an impedance matching transformer Yhaving windings for connecting said leads to a measuring circuit and being disposed in said head portion; said rod and said tube consisting of metal having a temperature coeiiicient of resistance of a lower order of magnitude than said resistance and transformer windings so that the measuring error of the thermometer is substantially limited to that caused by energy dissipation in said transformer; and temperature-responsive means arranged for substantially compensating such error and disposed in said head portion in proximity to said transformer so as to assume substantially the same temperature as said transformer.

GEORGE 'KEINATH REFERENCES CITED The following references are of record in the ille of thispatent:

UNITED STATES PATENTS Number Name Date 1,144,776 Northrup June 29, 1915 1,208,756 DeKhotinsky Dec. 19, 1916 1,379,526 Childs May 24, 1921 1,741,231 Grondahl Dec. 31, 1929 1,910,866 Van Nostrand May 23, 1933 2,049,285 Bauer July 29, 1936v 2,097,226 Miyazaki' Oct. 26, 1937 2,149,448 Lederer et al Mar. 7, 1939 2,219,939 Rich Oct. 29, 1940 2,245,700 Mounce June 17, 1941 2,271,975 Hall Feb. 3, 1942 2,297,780 Pugh, Jr Oct. 6, 1942 2,365,706 Keinath Dec. 26, 1944 FOREIGN PATENTS Number Country y Date 465,614 Great Britain May 10, 1937 

